<h2>DESCRIPTION</h2>

Aerial photographs may be either vertical or oblique. Vertical photographs
can be truly vertical (nadir), or slightly tilted (less than 3 degree
from the vertical). Usually aerial photos are tilted to some degree. We
refer to the term <i>vertical photograph</i> up to a tilt of 3 degree.
<br>
Oblique aerial photographs are purposely taken with an
angle between 3 and 90 degree from the nadir direction.
<p>
<b>The use of <em>i.ortho.init</em> (menu 6) is only required when rectifying a
tilted or oblique aerial photo.</b>
<p>
<em>i.ortho.init</em> creates or modifies entries in a camera initial exposure
station file for imagery group referenced by a sub-block.  These entries
include: the (XC,YC,ZC) standard (e.g. UTM) approximate coordinates of the camera exposure
station; initial roll, pitch, and yaw angles (in degrees) of the cameras
attitude; and the <em>a priori</em> standard deviations for these
parameters. During the imagery program, <em>i.photo.rectify</em>, the initial camera
exposure station file is used for computation of the ortho-rectification
parameters.  If no initial camera exposure station file exist, the default
values are computed from the control points file created in <em><A
HREF="i.photo.2target.html">i.photo.2target</a></em>.


<p>

The following menu is displayed:
<pre>
        Please provide the following information

	INITIAL XC: Meters                __________
	INITIAL YC: Meters                __________
	INITIAL ZC: Meters                __________
	INITIAL omega (pitch) degrees:    __________
	INITIAL phi  (roll) degrees:      __________
	INITIAL kappa  (yaw) degrees:     __________

	Standard Deviation XC: Meters     __________
	Standard Deviation YC: Meters     __________
	Standard Deviation ZC: Meters     __________
	Std. Dev. omega (pitch) degrees:  __________
	Std. Dev. phi  (roll) degrees:    __________
	Std. Dev. kappa  (yaw) degrees:   __________

        Use these values at run time? (1=yes, 0=no)

     AFTER COMPLETING ALL ANSWERS, HIT &lt;ESC&gt; TO CONTINUE
                  (OR &lt;Ctrl-C&gt; TO CANCEL)
</pre>

<p>
The INITIAL values for (XC,YC,ZC) are expressed in standard (e.g. UTM) coordinates, and
represent an approximation for the location of the camera at the time of 
exposure.

<ul>
<li> X: East aircraft position;
<li> Y: North aircraft position;
<li> Z: Flight altitude above sea level
</ul>

<p>

The INITIAL values for (omega,phi,kappa) are expressed in degrees, and
represent an approximation for the cameras attitude  at the time of 
exposure. 

<ul>
<li> Omega (pitch): Raising or lowering of the aircraft's front (turning
  around the wings' axis);
<li> Phi (roll): Raising or lowering of the wings (turning around the
  aircraft's axis);
<li> Kappa (yaw): Rotation needed to align the aerial photo to true north:
  needs to be denoted as +90 degree for clockwise turn and -90 degree for
  a counterclockwise turn.
</ul>

<p>
If ground control points are available, the INITIAL values are iteratively 
corrected. This is particularl useful when the INITIAL values are rather 
rough estimates.

<p>

The standard deviations for (XC,YC,ZC) are expressed in meters, and
are used as <em>a priori</em> values for the standard deviations used in 
computation of the ortho rectification parameters. Higher values improve 
the refinement of the initial camera exposure. As a rule of thumb, 5% 
of the estimated target extents should be used.
<p>

The standard deviations for (omega,phi,kappa) are expressed in degrees, and
are used as <em>a priori</em> values for the standard deviations used in 
computation of the ortho rectification parameters. As a rule of thumb, 
2 degrees should be used.

<p>
If <i>Use these values at run time? (1=yes, 0=no)</i> is set to 0, the
values in this menu are not used.

<h2>SEE ALSO</h2>

<em>
<a href="i.ortho.photo.html">i.ortho.photo</a>,
<a href="i.photo.2image.html">i.photo.2image</a>,
<a href="i.photo.2target.html">i.photo.2target</a>,
<a href="i.ortho.elev.html">i.ortho.elev</a>,
<a href="i.ortho.camera.html">i.ortho.camera</a>,
<a href="i.ortho.transform.html">i.ortho.transform</a>,
<a href="i.photo.rectify.html">i.photo.rectify</a>
</em>



<h2>AUTHOR</h2>

Mike Baba,  DBA Systems, Inc.
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